This application relates to magnetic disc drives and more particularly to a method of encoding and decoding head disc assembly serial numbers in servo bursts.
Disc drives are data storage devices that store digital data in magnetic form on concentric tracks on a rotating storage medium on a hard disc. A read/write transducer, e.g. a magnetoresistive read/write head, is typically used to transfer data between a desired track and an external environment. The head is typically mounted on a gimbal assembly at the end of an elongated actuator arm which pivots about an axis parallel but spaced from the spin axis of the hard discs. The actuator thus allows the head to move back and forth in an arcuate fashion between an inner radius and an outer radius of the discs. The actuator arm is driven by a control signal fed to a voice coil motor (VCM) mounted at the rear end of the actuator arm. During a write operation, data is written onto a track and during a read operation the head senses the data previously written on the disc track and transfers the information to the external environment. Critical to both of these operations is the accurate locating of the head over the center of the desired track.
A servo system is used to control the movement of the head above the disc using servo signals read from a disc surface in the disc drive. The servo signals generally indicate the present position of the head with respect to the disc, i.e., the current track position. The servo system uses the sensed information to determine how to optimally move the head to a position centered above the desired track. The servo system then delivers a control signal to the VCM to rotate the actuator to position the head over the desired track.
Servo information is typically stored in a disc drive apparatus in one of two ways: sectored servo and dedicated servo. In a sectored servo system, servo information is interspersed with user data on the disc surface. The servo information is stored in arcuate sectors of radial segments or wedges on each track of the disc surface. These segments are interspersed around the track between stored user data on the track. As the disc rotates beneath the head, the head periodically samples the servo sectors to obtain the servo information for the servo system. In a dedicated servo system, the servo information is stored on a separate dedicated disc surface that contains no user data. In this system, servo information is constantly available to the servo control system. In both systems, servo information, in the form of servo bursts which are stored on the discs following drive assembly and prior to drive use to store and retrieve data, is written during a servo track writing process. The servo track writing process, is used to tell the system whether the heads are correctly located over the desired track. Each servo burst typically contains the following sections: an automatic gain control (AGC) segment, a GAP segment, a prefix segment, a gray code segment, and four track following bursts A, B, C, and D.
The current manufacturing process, and some drive customers, require the serial number of the head disc assembly (HDA) to be electronically stored and retrievable on demand. Typically, a bar code label is attached to the outer surface of the HDA that uniquely identifies the drive with a serial number. This label may be optically read during the servo track writing (STW) process and written in the data field of the servo burst. However, this approach is expensive and takes additional time during the servo track writing process. An EEPROM or other non-volatile memory can be used to store the serial number, once read or manually entered. However, this approach requires additional non-volatile memory. The information could be interleaved into the normal gray code and servo frame space, but since modulo gray code needs all servo bursts in order to track follow, it is not feasible.
Against this backdrop the present invention has been developed. The present invention is a method of encoding and also retrieving information, and in particular, head disc assembly serial number information in the servo information recorded as part of the servo track information. Specifically, the HDA serial number information is written to the disc as part of the STW process in between the normal servo frames. The serial number is preferably written on preexisting unique system (non-user) tracks in-between normal servo frames or wedges. By increasing the sampling interval of the servo information, i.e., by over-sampling the servo information, these unique (non-user) tracks may be read, and the information such as the HDA serial number may then be retrieved. More specifically, by writing the serial number exactly in the middle of two normal servo frames, the information can be retrieved by 2xc3x97over-sampling.
The serial number, up to 16 characters long, typically made of American Standard Code for Information Interchange (ASCII) characters, is stored in an encoded fashion on the disc. Typically the first and last 8 tracks on the disc are not generally used for user data as they are very close to the inner and outer diameter guard bands. Therefore these bands or tracks are designated the xe2x80x9cover-samplexe2x80x9d tracks, and the gray coded HDA serial number information is written in gray code in servo bursts located exactly between the normal burst sectors. The encoding is done in the following manner. First, each 8 bit ASCII HDA serial number character is converted to 9 bits with even parity appended. Second, the 9 bit parity appended ASCII character is encoded in the appropriate gray code. By using the 9 bits of ASCII as an index into the encode table of run length limited gray codes stored in a ROM table, the gray code information is radially coherent across multiple tracks on the disc. Third, the encoded serial number is written, one gray encoded character at a time, per over-sample frame on the disc, starting at physical index (servo sector 0). Fourth, the writing process is repeated around the disc such that multiple copies of the serial number are written on each of the over-sample tracks. For example, if there are 16 characters in the serial number, with the last character being a termination character gray encoded zero, and there are 96 normal servo frames or wedges on the disc, then there will be 6 complete copies of the HDA serial number on each over-sample track on the disc and thus 6 copies of each character on each track.
The decode process is the opposite of the encode process, with the decode taking place during track following operation while the drive is following, over the specified xe2x80x9cover-samplexe2x80x9d track location. For example, first, a user request is lodged to retrieve the HDA serial number. This request is typically made in software, namely certification routines and diagnostic routines. The actuator head is positioned on one of the non-user over-sample tracks and the servo bursts are 2xc3x97 over-sampled. Second, each gray code retrieved from an over-sampled burst is decoded, i.e. converted to a 9 bit ASCII character. Third, each decoded 9 bit character is converted back to an 8 bit ASCII serial number character and stored for subsequent display or use. This process is repeated for each over-sample burst around the disc until a termination character is sensed, signifying the end of the HDA serial number. Over-sampling is then ceased and control is returned to normal burst processing. If errors are encountered during the read operation on the over-sample track, the following events may occur: All copies of the serial numbers on a given track are tried. If the error persists, the head is switched to another track and the decode is tried again to decode another copy of the serial number. This process is repeated until the entire serial number is successfully retrieved. With multiple copies of the serial number encoded on these tracks, the likelihood of successful retrieval is maximized. The number thus retrieved may be subsequently stored on the data portion of the disc and retrieved in a conventional manner, or may be reread each time a diagnostic or certification routine makes the request.
These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.